Writing method for magnetic random access memory using a bipolar junction transistor

ABSTRACT

A writing method for a magnetic random access memory (MRAM) using a bipolar junction transistor is disclosed. The writing method is for use with the MRAM using the bipolar junction transistor including: a semiconductor substrate serving as a base of the bipolar junction transistor; an emitter and a collector of the bipolar junction transistor formed in an active region of the semiconductor substrate; an MTJ cell positioned in the active region between the emitter and the collector, separately from the emitter and the collector by a predetermined distance; a word line formed on the MTJ cell; a bit line connected to the collector; and a reference voltage line connected to the emitter. The writing method for the MRAM using the bipolar junction transistor writes data by applying current from the emitter to the collector, and changing a magnetization direction of a free ferromagnetic layer of the MTJ cell by a magnetic field generated due to the current. The magnetization direction can be varied in a shorter distance than a distance between the MTJ cell and the bit line, thereby reducing current consumption. It is thus possible to improve a property and reliability of the semiconductor device.

BACKGROUND

1. Technical Field

The present invention relates to a writing method for a magnetic randomaccess memory (abbreviated as ‘MRAM’) including a bipolar junctiontransistor and, in particular, to an improved writing method for an MRAMhaving a higher speed than the static random access memory (SRAM),integration as high as the dynamic random access memory (DRAM), and aproperty of a nonvolatile memory such as a flash memory.

2. Description of the Related Art

Most of the semiconductor memory manufacturing companies have developedan MRAM using a ferromagnetic material and consider the MRAM as one ofthe next generation memory devices.

The MRAM is a memory device for reading and writing information byforming multi-layer ferromagnetic thin films, and sensing currentvariations according to magnetization directions of the respective thinfilms. The MRAM has a high operating speed, low power consumption andhigh integration density, all of which are made possible by the specialproperties of the magnetic thin film. MRAM devices may performnonvolatile memory operations that are presently carried out using aflash memory.

The MRAM embodies a memory device using either a giant magneto resistive(abbreviated as ‘GMR’) phenomenon or a spin-polarizedmagneto-transmission (SPMT) phenomenon generated when the spininfluences electron transmission.

MRAM devices based on the GMR phenomenon utilize the phenomenon thatresistance vanes remarkably when spin directions are different in twomagnetic layers having a non-magnetic layer therebetween, which is how aGMR magnetic memory device is implemented.

MRAM devices based on the SPMT approach utilize the phenomenon thatlarger current transmission is generated when spin directions areidentical in two magnetic layers having an insulating layertherebetween, which is how a magneto-transmission junction memory deviceis implemented.

However, MRAM research is still in its early stage. Today, MRAM researchis mostly concentrated on the formation of multi-layer magnetic thinfilms and is less focused on research pertaining to unit cell structureand to peripheral sensing circuits.

FIG. 1 is a cross-sectional diagram illustrating a conventional MRAMusing a bipolar junction transistor. As shown in FIG. 1, the MRAMincludes a semiconductor substrate 111 used as a base of the bipolarjunction transistor, an emitter 113 a and a collector 113 b formed in anactive region of the semiconductor substrate 111 according to an implantprocess and a stacked structure of an MTJ cell 121 and a word line 123positioned in the active region between the emitter 113 a and thecollector 113 b, spaced apart from the emitter 113 a and the collector113 b by a predetermined distance. The emitter 113 a and the collector113 b are formed according to an implant process using a mask. The MRAMalso includes a bit line 135 connected to the collector 113 b and areference voltage line 127 connected to the emitter 113 a. Here, a gateoxide film (not shown) is not formed below the MTJ cell 121 or word line123.

The MTJ cell 121 comprises a stacked structure including a freeferromagnetic layer 115, a tunnel barrier layer 117 and a pinnedferromagnetic layer 119. At this time, at least three multiple datarecording states including ‘0’ or ‘1’ can be obtained in one cell of thememory device. These states are created by setting up a magnetizationdirection of the free ferromagnetic layer 115 to be identical to oropposite to a magnetization direction of the pinned ferromagnetic layer119 or to have a predetermined angle.

The bit line 135 is connected to the collector 113 b through aconnection line 129 and a contact plug 133.

With reference to FIG. 1, a conventional method for fabricating the MRAMwill now be described. A mask layer (not shown) for exposing apredetermined region of the emitter and collector is formed in theactive region of the semiconductor substrate 111. The emitter 113 a andthe collector 113 b are formed by implanting an impurity into thesemiconductor substrate 111, and the mask layer is then removed.

The stacked structure of the pinned ferromagnetic layer 115, the tunnelbarrier layer 117 and the free ferromagnetic layer 119 is formed overthe resultant structure to form the MTJ cell.

The MTJ cell 121 of an island type is formed by patterning the stackedstructure of the pinned ferromagnetic layer 115, the tunnel barrierlayer 117 and the free ferromagnetic layer 119 according to aphotolithography process using an MTJ cell mask (not shown).

A conductive layer for a word line is formed over the resultantstructure, and patterned to form the word line 123 according to aphotolithography process using a word line mask (not shown). Thisprocedure forms the stacked structure of the MTJ cell 121 and the wordline 123. Here, the word line 123 may comprise a mask insulating film atits upper portion, which results in an improved insulating property.

The stacked structure of the MTJ cell 121 and the word line 123 isformed in the active region between the emitter 113 a and the collector113 b. The MTJ cell 121 and the word line 123 are spaced apart from theemitter 113 a and the collector 113 b by a predetermined distance.

Thereafter, a first interlayer insulating film 125 is formed toplanarize the top surface of the resultant structure. Here, the firstinterlayer insulating film 125 is planarized to expose the upper portionof the word line 123.

The connection line 129 and the reference voltage line 127 are formed torespectively contact the emitter 113 a and the collector 113 b throughthe first interlayer insulating film 125.

A second interlayer insulating film 131 is formed over the resultantstructure, and evenly etched to planarize the top surface thereof.

A bit line contact plug 133 is formed to contact the connection line 129through the second interlayer insulating film 131.

Here, the connection line 129 is exposed by etching the secondinterlayer insulating film 131 according to a photolithography processusing a bit line contact mask (not shown). After etching, a conductivelayer for a bit line contact plug is deposited to contact the connectionline 129, and evenly etched to expose the second interlayer insulatingfilm 131, thereby forming the bit line contact plug 133.

The bit line 135 is formed to contact the bit line contact plug 133.

Here, the bit line 135 is formed by depositing and patterning aconductive layer for a bit line contacting the bit line contact plug133.

The operation of the MRAM will now be described with reference to FIG.1.

A data write operation is performed by applying current to the word line123 and the bit line 135 regardless of the transistor.

When the current is applied to the word line 123, the current does notflow toward the transistor due to resistance elements of the tunnelbarrier layer 117 formed between the pinned ferromagnetic layer 115 andthe free ferromagnetic layer 119 in the MTJ cell 121. Rather, thecurrent flows toward the word line 123.

The current applied to the bit line 135 does not flow from the collectorof the bipolar junction transistor to the base or emitter thereof, butflows through the bit line itself.

An amount and direction of the current in the word line 123 and the bitline 135 crossing each other in a vertical direction or at apredetermined angle, are controlled. The amount and direction of thecurrent sets up the magnetization direction of the free ferromagneticlayer 119 of the MTJ cell 121 in a desired direction, thereby performingthe data write operation.

After the data write operation, the magnetization direction of the freeferromagnetic layer 119 of the MTJ cell 121 is set to be identical to oropposite to the magnetization direction of the pinned ferromagneticlayer 115. Alternatively, the MTJ cell 121 may be set up at apredetermined angle with respect to the magnetization direction.

The MTJ resistance varies according to the angle between the freeferromagnetic layer 119 and the pinned ferromagnetic layer 115. The datawrite operation is performed using the MTJ resistance values.

In a data read operation, a voltage is applied to the bit line 135 andthe word line 123. Here, the current does not flow in the bit line 135or the word line 123.

When the current flows through the MTJ cell 121 due to the voltageapplied to the word line 123, a voltage drop appears due to theresistance of the MTJ cell 121, thereby allowing the voltage applied tothe input terminal of the transistor, namely the semiconductor substrate111, to vary according to the resistance value of the MTJ cell 121.

Therefore, when the voltage and current applied to the input terminalare varied, a signal at the collector 113 b varies. A reading of theinformation is performed by sensing the signal at the bit line connectedto the output terminal of the transistor.

As described above, the conventional method of writing information tothe MRAM using the bipolar junction transistor inverts the magnetizationdirection of the MTJ free ferromagnetic layer according to a magneticfield generated due to the current flowing through the word line 123 andthe bit line 135. As a result, the current must be applied to the bitline 135 at a location that is far from the MTJ cell 121 to obtain adesired magnetic field intensity. The fact that the current must beapplied to the bit line a significant distance from the MTJ cell resultsin high current consumption when writing information to the MRAM device.

SUMMARY OF THE DISCLOSURE

In accordance with one aspect of the disclosure, an operation method foruse with a magnetic random access memory (MRAM) using a bipolar junctiontransistor is disclosed. The MRAM may include a semiconductor substrateserving as a base of the bipolar junction transistor, an emitter and acollector of the bipolar junction transistor respectively formed on afirst portion and a second portion of the semiconductor substrate,wherein the emitter and the collector are spaced apart and a MTJ cellformed in the semiconductor substrate between the emitter and thecollector. The MRAM may also include a word line formed on the MTJ cell,a bit line connected to the collector and a reference voltage lineconnected to the emitter. The method may include applying current fromthe emitter to the collector and changing a magnetization direction andangle of a free ferromagnetic layer of the MTJ cell using a magneticfield generated by the applied current, thereby writing data.

In accordance with another aspect of the disclosure, a magnetic randomaccess memory (MRAM) using a bipolar junction transistor is disclosed.The MRAM may include a semiconductor substrate serving as a base of thebipolar junction transistor, an emitter and a collector of the bipolarjunction transistor respectively formed on a first portion and a secondportion of the semiconductor substrate, wherein the emitter and thecollector are spaced apart and a MTJ cell formed in the semiconductorsubstrate between the emitter and the collector. The MRAM may alsoinclude a word line formed on the MTJ cell, a bit line connected to thecollector and a reference voltage line connected to the emitter. TheMRAM further includes a current source connected to the emitter forapplying current from the emitter to the collector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram illustrating a conventional writingmethod for an MRAM using a bipolar junction transistor; and

FIG. 2 is a cross-sectional diagram illustrating the disclosed writingmethod for an MRAM using a bipolar junction transistor.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

A writing method for a magnetic random access memory including a bipolarjunction transistor will now be described in detail with reference tothe accompanying drawings.

FIG. 2 is a cross-sectional diagram illustrating the disclosed writingmethod for the MRAM including the bipolar junction transistor. The MRAMstructure of FIG. 2 has the same or substantially the same constitutionas the MRAM of FIG. 1.

The operation of the MRAM is now explained with reference to FIG. 2. Adata write operation is performed by applying a large amount of currentfrom an emitter 213 a of the bipolar junction transistor to a collector213 b through a base, namely through a semiconductor substrate 211.

The data write operation is executed by generating a magnetic field byapplying the current to the collector 213 b, and controlling the currentto set a magnetization of a free ferromagnetic layer 219 in an MTJ cell221 in a desired direction.

After the data write operation, the magnetization direction of the freeferromagnetic layer 219 is identical to or opposite to a magnetizationdirection of a pinned ferromagnetic layer 215. Alternatively, themagnetization of the free ferromagnetic layer 219 may have apredetermined angle with respect to the pinned ferromagnetic layer 215.

A resistance value of the MTJ cell 221 varies according to an anglebetween the free ferromagnetic layer 219 and the pinned ferromagneticlayer 215, which is used to perform the data write operation.

On the other hand, in a data read operation, a voltage is applied to abit line 235 and a word line 223. When current flows through the MTJcell 221 due to the voltage applied to the word line 223, the voltagedrop appears due to the resistance of the MTJ cell 221, thereby allowingthe voltage applied to the input terminal of the transistor, namely thesemiconductor substrate 211, to vary according to the resistance valueof the MTJ cell 221.

When the voltage and current applied to the input terminal are varied, asignal appearing at the collector 213 b is varied. A reading ofinformation is performed by sensing the signal at the bit line connectedto the output terminal of the transistor.

As discussed earlier, the write operation is performed by forming theMTJ cell 221 as the input terminal of the bipolar junction transistor,applying the current from the emitter of the bipolar junction to thecollector through the base, and controlling the magnetization directionof the free ferromagnetic layer of the MTJ cell by using the current.The magnetization direction of the free ferromagnetic layer can becontrolled by using a minimal amount of current, thereby improving aproperty and reliability of the device.

The disclosed writing method for writing to a magnetic random accessmemory (MRAM) using a bipolar junction transistor efficiently performs awrite operation with a small amount of current. Only a small amount ofcurrent is required because a magnetic field is generated using currentthat is excited in an adjacent position and that flows from an emitterto a collector of the bipolar junction transistor. In principle, thewrite and read operations of the device may be performed with a smallamount of current by adjusting the magnetization direction of the freeferromagnetic layer using current flowing from the emitter to thecollector.

Although certain apparatus constructed in accordance with the teachingsof the invention have been described herein, the scope of coverage ofthis patent is not limited thereto. On the contrary, this patent coversall embodiments of the teachings of the invention fairly falling withinthe scope of the appended claims either literally or under the doctrineof equivalents.

What is claimed is:
 1. An operation method for use with a magneticrandom access memory (MRAM) using a bipolar junction transistorcomprising: a semiconductor substrate serving as a base of the bipolarjunction transistor; an emitter and a collector of the bipolar junctiontransistor respectively formed on a first portion and a second portionof the semiconductor substrate, wherein the emitter and the collectorare spaced apart; a MTJ cell formed in the semiconductor substratebetween the emitter and the collector; a word line formed on the MTJcell; a bit line connected to the collector; and a reference voltageline connected to the emitter, the method comprising: applying currentfrom the emitter to the collector; and changing a magnetizationdirection and angle of a free ferromagnetic layer of the MTJ cell usinga magnetic field generated by the applied current, thereby writing data.2. A magnetic random access memory (MRAM) using a bipolar junctiontransistor comprising: a semiconductor substrate serving as a base ofthe bipolar junction transistor; an emitter and a collector of thebipolar junction transistor respectively formed on a first portion and asecond portion of the semiconductor substrate, wherein the emitter andthe collector are spaced apart; a MTJ cell formed in the semiconductorsubstrate between the emitter and the collector; a word line formed onthe MTJ cell; a bit line connected to the collector; and a referencevoltage line connected to the emitter, the magnetic random access memoryfurther comprising: a current source connected to the emitter forapplying current from the emitter to the collector.